The present invention relates generally to refrigeration control systems. More particularly, the invention relates to a control technique that senses refrigerant energy and uses the sensed refrigerant energy as a control parameter in a control system that strives to maintain the sensed refrigerant energy in a state of chaotic or random fluctuation. Using this technique very precise control of the refrigerant liquid-vapor phase transition is achieved, and the entire length of the evaporator coil may be used without risk of flooding the compressor.
Most modern-day refrigeration systems (e.g. refrigerators, freezers, refrigerated display cases, air-conditioners and heat pumps) use a form of closed-loop feedback control in which the temperature of the superheated refrigerant is held constant.
In a heat pump refrigeration system liquid refrigerant is introduced under pressure into a heat exchanger or evaporator coil, typically configured as a long serpentine conduit, often with external fins to increase the heat exchange surface area. A compressor or pump pressurizes the refrigerant and causes it to flow through the evaporator coil. As it flows through the evaporator coil, the refrigerant absorbs heat energy through the sidewalls of the heat exchanger. In a properly operating system the refrigerant absorbs heat continuously throughout its traverse through the heat exchanger, so that the heat energy of the refrigerant exiting the heat exchanger is greater than the heat energy of the refrigerant entering the heat exchanger.
As heat energy continues to accumulate in the refrigerant a phase change eventually occurs. When sufficient heat has been absorbed, the liquid refrigerant is converted to vaporous refrigerant. At the microscopic level, this transition from liquid phase to vapor phase does not occur instantaneously. Rather, there is a transition region characterized by a mixture of refrigerant in the liquid phase and refrigerant in the vapor phase occurring simultaneously. Eventually, however, when enough heat has been added, all of the refrigerant enters the vapor phase. When this occurs, the refrigerant is said to have entered the superheat domain or superheat region.
Conventional refrigeration systems attempt to monitor the temperature of the refrigerant within the superheat region, as a way of controlling the refrigeration cycle. If the measured superheat temperature is too low, then the control system reduces the flow of refrigerant. Conversely, if the measured superheat temperature is too high, the control system increases the flow of refrigerant. There is a significant problem in using superheat temperature as a control parameter in this fashion. The transition to the vapor phase that marks the beginning of the superheat region is not positionally stable. Depending on the instantaneous load or demand on the system, the transition point into the superheat region shifts position unpredictably. This makes it very difficult to reliably sense the instantaneous superheat temperature using a positionally fixed sensor, such as a thermistor placed in thermal contact with the sidewall of the refrigerant conduit. Because the superheat region fluctuates unpredictably, sometimes the superheat region will be positioned directly over the fixed sensor, and sometimes the superheat region will be upstream or downstream of the fixed sensor. Conventional refrigeration systems address this unpredictability by placing the sensor where it is assured to avoid the fluctuating region altogether. Conventional systems use a threshold temperature that is known to be well above the unpredictable range, so that the fixed sensor is guaranteed to always sense the superheat region. The disadvantage of using this technique is that the refrigerant makes its transition to the superheat region while it is still in the heat exchanger, thereby reducing the heat exchanger's efficiency.
The present invention takes a completely different approach. Rather than striving to avoid the unstable superheat region, the present invention is designed to seek it. By way of introduction, the present invention provides a system for controlling a refrigeration cycle of the type having a fluid refrigerant that changes state between a liquid phase at a first energy and a liquid phase at a second energy. The system comprises a flow control system for regulating the flow of refrigerant and a sensor system positioned for detecting a parameter indicative of refrigerant energy. The invention further employs a recognition system coupled to the sensor system for discriminating between an energy associated with a predominantly liquid phase and an energy associated with a predominantly vapor phase. The invention further comprises a control system, coupled to the recognition system and to the flow control system, for operating the flow control system so that the energy of the refrigerant at the sensor position fluctuates alternately between an energy associated with the predominantly liquid phase and an energy associated with the predominantly vapor phase.
One of the advantages of the invention is that the entire heat exchanger is used efficiently through a control strategy that maintains the refrigerant within the heat exchanger in a predominantly liquid phase. In addition, the control system of the invention also prevents refrigerant in the predominantly liquid phase from entering or flooding the compressor.
For a more complete understanding of the invention, its objects and advantages, refer to the following specification and to the accompanying drawings.